Impact ionization has been known for several years. U.S. Pat. No. 4,432,075 to B. Eitan and U.S. Pat. No. 4,821,236 to Hayashi et al. describe an EEPROM transistor adjacent to a charge generator, creating a substrate current near the EEPROM, creating excess charge or holes, resembling space charge, near subsurface electrodes of the EEPROM. Assume that the holes are generated and accelerated toward one of the electrodes of the EEPROM. Resulting secondary electrons are sufficiently energetic to penetrate gate oxide over the substrate and become injected into a conductive floating gate. For EEPROMs, the floating gate becomes charged by band-to-band tunneling, a situation which eliminates the need for a control gate over the floating gate. It is known that EEPROMs using impact ionization require lower voltages for programming and erasing compared to conventional EEPROMs that can require 12 volts or more for programming.
U.S. Pat. No. 4,334,292 discloses a non-volatile memory cell employing at least two bit lines for programming and reading the cell. Charge injector regions are employed to reduce voltage supply levels for the memory cell.
U.S. Pat. No. 4,821,236 shows a subsurface injector region that generates charge for storage on a floating gate of a non-volatile memory cell. See also U.S. Pat. No. 6,125,053 regarding impact ionization.
One of the problems encountered in manufacturing EEPROMs is generating the high voltages required for programming. Where only low voltages are available, charge pumps, or the like, are typically employed to supply internal programming voltages. Charge pumps occupy valuable die area and require ancillary timing circuits for operating switches associated with the charge pumps. In turn, the timing circuits can require adjustment, needing other ancillary circuits. Similarly, circuits for reading EEPROMs require high voltages on control gates to drive charge from floating gates. Voltage must be high enough to obtain sufficient current flow that can be read. Once again, voltage increasing circuits must be used where only low voltage power supplies are employed.
An object of the invention was to devise an EEPROM architecture operating on very low voltages, yet not employing charge pumps or the like.